Fabric dyed with indigo is chiefly so-called denim fabric, which is a coarse cotton fabric mainly used for the production of so-called blue jeans, that is to say jeans and other articles of clothing based on denim fabrics, such as, for example, shirts, jackets, skirts and the like. These articles of clothing are produced by special processes characteristic of this type of clothing.
For the production of dyed textiles, the web of material on which they are based is usually dyed continuously or discontinuously. Only after this process is the web of material further processed by an expedient finishing treatment and finally made up, usually in another plant.
In exceptional cases, such as, for example, in the case of short-lived fashion articles (T-shirts), the undyed, made-up article of clothing can also be subsequently dyed, according to requirements. Piece dyeing is referred to here.
In the case of jeans fabrics (denims), on the other hand, the dyeing process is carried out in a very early production stage. The so-called "warp" is thus already sized and dyed in a combined working operation before the weaving process. "Size dyeing" is referred to here. This warp is then woven with an undyed weft thread, from which comes the characteristic denims fabric appearance. For reasons of cost, starch or starch derivatives are usually used for the sizing.
A wide range of the most diverse color shades and, depending on the type of fiber, a characteristic choice of classes of dye are available for textile dyeing.
In contrast, conventional jeans fabrics are usually produced from cotton and dyed blue.
There are indeed denim articles in the most diverse color shades, but conventional jeans fabric is blue. "Blue jeans" is a generic term firmly anchored in the language. Also traditionally, jeans fabrics must be dyed with indigo. Indigo is a natural dye, although nowadays it is chiefly prepared synthetically.
As already mentioned, the finishing of a textile substrate is usually carried out by the textile processor after dyeing. Only then are the goods sent for making up.
In the case of jeans fabrics, however, the fabric in the loom state is already made up to the article of clothing. "Processing" is carried out only during washing of the already-sewn pieces in plants which are particularly specialized for this, the so-called "jeans fabrics washers". Special drum washing machines are usually used for this. The inflexible and rigid pieces are first carefully softened to avoid crease folds, and troublesome chemicals, such as alkali and excess dye, are washed off.
After an optimum pH has been established, the pieces are then enzymatically desized, that is to say the starch size is broken down by amylase enzymes into soluble fragments and these are rinsed out. As a result, the fabric loses at least some of its stiffness.
In accordance with current fashion, a considerable percentage of the color applied is then removed by bleaching, until the article of clothing is no longer dark blue but appears in defined medium to light blue shades. This bleaching is predominantly effected by chlorination. Alternative processes, such as, for example, reductive bleaching (sugar wash) are also known.
If required by fashion, washing with lava stones is also carried out (so-called "stone-wash" process), which imparts to the trousers damaged by abrasion the so-called "worn-out look".
Recently, the use of cellulose enzymes in the washing has been promoted. However, these enzymes attack the cotton of the fabric itself and therefore make it softer.
Textile softeners are additionally added, however, to high-quality jeans fabrics while these are still in the rinsing bath, in order to impart to them a particularly good and soft handle.
Since the softeners for jeans clothing are applied in the rinsing bath of the washing machine, they must have a certain tendency to be absorbed substantively on to the goods from a long liquor. The products based on dispersions of polyethylene or wax and softeners based on fatty acid polyglycol ethers which are currently used are not optimum in the criteria of "soft handle" and absorption capacity. Cationic fatty softeners which are capable of absorption have even less favorable properties.
The use of amino-functional silicones as softeners would be desirable per se, since these substances result in a particularly good soft handle and, owing to their cationic charge, also have substantivity with respect to the negatively charged cotton. There is unfortunately--quite rightly--an aversion to silicone softeners in jeans fabrics: Jeans goods often remain in shops for a relatively long time before being sold. When stored in a stack, only the edges are exposed here to the direct action of the atmosphere. Air pollutants, such as ozone or nitrogen oxide (NO.sub.x), however, can damage the blue indigo dye, i.e. oxidize it to yellow isatin.
Jeans can therefore lose their color, specifically at edges in storage. This "ozone fading" makes these goods unsellable, especially since it occurs only locally. There is agreement in technical circles that although silicone softeners do not cause this ozone fading, they certainly promote and intensify it. The silicone softeners, which are actually particularly effective for this intended use, are therefore rejected. See also Melliand Textilberichte, "Vergilbung von indigohaltiger Jeanswear" [Yellowing of indigo-containing jeanswear] 11/96, page 786 et seq., 1996, where the influence of the soft handle agent and storage time is stated on page 787 under 6:
"Of the soft handle agents investigated (various fatty acid condensation products, and amino-functional polysiloxane and a handle agent based on polyurethane) none causes intrinsic yellowing under the action of harmful gas. However, silicone-containing products accelerate the degradation of indigo by harmful gas. Individual fatty acid condensation products significantly reduce the yellowing of jeanswear. The longer jeanswear is exposed to the action of harmful gas, the greater the probability of the development of irreparable lightening and yellowing. According to observations in practice, there is a yellowing maximum which is stationary under lasting lightening." PA1 R is preferably identical or different, optionally substituted hydrocarbon radicals or hydrocarbonoxy radicals having in each case 1 to 18 carbon atoms, hydrogen atoms or hydroxyl radicals, PA1 R' is preferably identical or different, SiC-bonded substituted hydrocarbon radicals containing polar groups, PA1 n is an integer having the value of 0, 1, 2 or 3, PA1 m is an integer having the value of 0, 1, 2 or 3 and the sum n+m has an average value of 1.8 to 2.2 and m is chosen such that the polyorganosiloxane contains at least one radical R'. PA1 R.sup.1 is preferably a divalent C.sub.1 - to C.sub.18 -hydrocarbon radical, PA1 R.sup.2 is preferably a hydrogen atom or an optionally fluorine-, chlorine- or bromine-substituted C.sub.1 - to C.sub.18 -hydrocarbon radical, PA1 a has the values 2, 3, 4, 5 or 6 and PA1 b has the values 0, 1, 2, 3 or 4. PA1 R.sup.1 is a divalent C.sub.2 - to C.sub.6 -hydrocarbon radical, PA1 R.sup.2 is a hydrogen atom or a methyl or cyclohexyl radical, PA1 a has the value 2 or 3 and PA1 b is the value 0 or 1. PA1 X is a linear, branched aliphatic, aromatic or mixed aliphatic-aromatic hydrocarbon radical, the carbon skeleton of which can be interrupted by divalent sulfuir, oxygen or carboxylic acid ester radicals, and PA1 p has the value 1 or 2. PA1 --(CH.sub.2).sub.10 --COOH, PA1 --CH.sub.2 CH(CH.sub.3)--COOH and PA1 --(CH.sub.2).sub.2 --S--CH.sub.2 --COOH. PA1 R" is preferably a linear or branched, saturated or unsaturated alkyl radical having on average 8 to 24 carbon atoms, preferably 8 to 16 carbon atoms, and Z.sub.o is preferably an oligoglycoside radical having on average o=1 to 10, preferably 1 to 5 hexose or pentose units, or mixtures thereof. PA1 a) 100 parts by weight of organopolysiloxanes which preferably contain polar groups on SiC-bonded hydrocarbon radicals and PA1 b) preferably up to 150 parts by weight of alkyl polyglycosides. PA1 Ozone concentration: about 200 mg/hour PA1 Flow rate: 2.0 l/hour PA1 Treatment time: 60 minutes at room temperature
Our own experiments have shown that this reduction is not based on prejudice. In fact, an intensification of the deviation in color of denim fabrics can also be found experimentally if these have been treated with commercially available silicone softeners before the action of ozone or NO.sub.x (Example 1).
This effect is practically independent of whether the softener is present in the form of a micro- or macroemulsion, and of what viscosity, amine number or structure the silicone has.
Surprisingly, it has now been found that it is not the silicone but the emulsifier usually used for the emulsification which is responsible for this ozone fading, regardless of whether it is based on fatty alcohol ethoxylate or alkylphenol ethoxylate (Example 2).
Silicones which, unusually in practice and purely for experimental purposes, were applied to the fabric as a solution in non-polar solvents have no effect-intensifying action. On the other hand, ethoxylated emulsifiers promote ozone fading.
This action seems to be largely independent of the hydrophobic radical (straight- or branched-chain fatty alcohol, alkylphenol), independent of the degree of ethoxylation (n=5-10) and independent of the manufacturer of the surfactant. Any differences in the Gaussian distribution, the process control and possibly the catalysis during the ethoxylation therefore do not seem to play a role.